A variety of technologies capable of nondestructively inspecting an internal structure of a sample have been developed over the recent years and utilized in many fields. One known technology of such a type is an optical coherence tomography (OCT) for obtaining a coaxial tomographic image of the sample with the use of a light beam having a short coherence length.
The OCT will hereinafter be outlined. The OCT involves the use of an optical measuring instrument including a light source for emitting a light beam having a short coherence length (on the order of several tens of .mu.m), an interferometer constructed of an optical multiplexer/demultiplexer, a movable reflection mirror and a scan system, and an analyzing system.
The short coherence length light emitted by the light source provided in the optical measuring instrument is guided to the optical multiplexer/demultiplexer constituting the interferometer, and separated into a beam of measurement light and a beam of reference light. The measurement light is guided to a sample (e.g., an eye) via the scan system for changing a position for guiding the measurement light to the sample, then reflected within the sample, and travel back to the optical multiplexer/demultiplexer via the scan system. On the other hand, the reference light is reflected by the reflection mirror moving back and forth in a distance range corresponding to a measuring range of the sample in a direction of the optical axis of the reference light, thereafter travels back to the optical multiplexer/demultiplexer, and is multiplexed by the optical multiplexer/demultiplexer with the reflected light from the sample. Incidentally, for facilitating a process in the analyzing system, the reflection mirror generally takes such a motion pattern that there exists a time zone in which the reflection mirror moves at a fixed velocity such that it returns to a starting point at a high velocity after moving at a fixed velocity from the starting point to an ending point of the distance range.
The analyzing system executes a process of obtaining a corresponding relationship between a position of the reflection mirror and a degree to which the light multiplexed by the optical multiplexer/demultiplexer is modulated (i.e., a process of obtaining optical characteristic data about several positions, having different depths, of a portion to which the measurement light is introduced), and stores a result of this process. When obtaining a sectional image perpendicular to the optical axis of the measurement light, the measurement light beams are introduced to the respective positions required to be measured by the scan system, and the analyzing system calculates and stores the optical characteristic data about the respective positions. Then, the analyzing system obtains plural pieces of optical characteristic data, and, based on these pieces of optical characteristic data, creates and displays the sectional image.
That is, the OCT-oriented optical measuring instrument utilizes the short coherence length light for distinguishing the light beam reflected in a specified position among a multiplicity of light beams simultaneously incident upon the optical multiplexer/demultiplexer and reflected in a multiplicity of positions having different depths within the sample. More specifically, as a result of being reflected in the positions having the different depths, the light beams which have reached simultaneously the optical multiplexer/demultiplexer are defined as short coherence length light beams with different demultiplexing times at which the optical multiplexer/demultiplexer has demultiplexed the measurement light as a basis. Therefore, what interferes with the reference light coming from the reflection mirror among those light beams is only the reflected light of the measurement light demultiplexed by the optical multiplexer/demultiplexer at the same time as that of the reference light, i.e., the light reflected in such a position that a length of an optical path of the measurement light is equal to a length of an optical path of the reference light. Then, a wavelength of the reference light is shifted due to the motion of the reference mirror, and hence the light multiplexed by the optical multiplexer/demultiplexer is the light modulated corresponding to a magnitude of the measurement light component representing an optical characteristic of the depth determined by the length (correlated to the position of the reference mirror) of the optical path of the reference light at that point of time within the sample. Therefore, the analyzing system analyzes a degree of modulating an intensity of the light multiplexed by the optical multiplexer/demultiplexer in connection with a position of the reflection mirror, thereby making it feasible to obtain the optical characteristic at the depth of the portion to which the measurement light is introduced. According to the OCT, the measurement based on the principle described above is repeated at respective points in the sample, thus obtaining two- and three-dimensional images of the sample.
Note that the OCT technology is exemplified in the form of a literature on pp.1178-1181 of "Optical Coherence Tomography", written by D. Huang et al., Science, 1991,254.
As obvious from the description given above, a spatial resolution of the OCT-oriented optical measuring instrument (which is hereinafter simply referred to as the optical measuring instrument), is basically determined by a coherence length of the light used for the measurement. Therefore, the measurement can be carried out with a higher spatial resolution than by other measuring technologies such as an ultrasonic measuring technology (a spatial resolution is on the order of 150 .mu.m when measuring at 10 MHz conceived as a general measurement condition) and a laser scan microscope technology (a spatial resolution is on the order to 200 .mu.m when measuring an eyeground).
The prior art optical measuring instrument is, however, a single channel type instrument capable of measuring only one point existing on the optical axis of the measurement light in the measurement at a certain time, and therefore requires much time for measuring a plurality of points having different depths. It must be a problem in terms of a cost performance that a long time is required for the measurement. Further, if difficult to maintain a measurement object sample in the same position for a long period of time as in the case of a living body sample, a problem in terms of a measuring accuracy might be induced. For example, when measuring an eyeball, it might happen that a relative positional relationship between the optical measuring instrument and the measurement object sample fluctuates due to a motion of the head of a subject and to a fixation micronystagmus. In the prior art optical measuring instrument, a comparatively long time is needed for finishing the measurement in a target range, and therefore, in the meantime, a fluctuation is seen in this positional relationship, with the result that optical characteristic data on positions other than the target position are frequently measured.